Large volume fluid intake systems are used for generating hydroelectric power, providing cooling water for manufacturing and power generation plants, providing irrigation and potable water supplies, and providing source water for desalinization plants. In the U.S. alone, these systems take in more than 200 billion gallons of fluid per day. Unfortunately, according to U.S. Environmental Protection Agency (EPA) estimates, these fluid intake systems remove billions of aquatic organisms from the water bodies in which they are used, including fish, crustaceans, shellfish, sea turtles, marine mammals, as well as a plethora of other aquatic life forms.
Eggs and larvae of fish (commonly referred to as ichthyoplankton) are particularly sensitive to large volume fluid intake systems because they have little or no swimming ability. Ichthyoplankton range in size from about 0.5 mm to greater than 1 mm in diameter, and normally reside in the upper 200 meters of the water column, where they drift passively in the prevailing currents.
Fluid intake systems negatively impact aquatic life in two major ways: entrainment and impingement. Entrainment is the circumstance where an aquatic organism is drawn into the intake system and subjected to the physical, mechanical, chemical, and thermal forces particular to the design and function of the fluid manipulation system in question. Impingement describes the circumstance where an aquatic organism is trapped against an; upstream physical barrier by the force of fluid flow entering the intake system, and usually occurs in situations where the intake system is screened. Most large volume fluid intake systems are screened to prevent entrainment of debris.
Mortality rates of ichthyoplankton that are either entrained or impinged is high, and may approach 100%. To minimize the impact of large volume fluid intake systems on aquatic ecosystems, Section 316(b) of the Clean Water Act mandates that large volume fluid intake systems, such as the water intake systems used in cooling power plants, reduce impingement levels by 80-95% and entrainment levels by 60-90%. EPA estimates suggest that Section 316(b) compliance will result in benefits to recreational and commercial fishing industries in excess of $100 million annually. Additionally, Section 316(b) compliance is likely to have a beneficial, although difficult to quantify, environmental effect by creating healthier and more robust aquatic ecosystems.
Rates of entrainment and impingement are affected by many factors. For example, both entrainment and impingement are affected by the pore size of the apparatus used to screen the intake system. There is a linear relationship between entrainment and pore size (i.e. entrainment rates increase as filter pore size increases), while there is an inverse relationship between impingement rates and pore size (i.e. impingement rates increase as filter pore size decreases).
Entrainment and impingement rates are also affected by several other factors including, but not limited to, the velocity of the fluid intake system (Vi) and the velocity of the source water body (Vw) that serves the fluid intake system. Under most circumstances, Vi has a constant value within the fluid intake system that is determined either by gravity or a pump. However, at the point of fluid intake, Vi interacts with the source water body in a complex manner whereby the value of Vi decreases with distance (d) from the point of fluid intake. As d increases relative to the point of fluid intake, it can be recognized that d will eventually reach a critical distance (dmax) where Vi is equal to Vw. In other words, an object in the source water body located outside of the dmax area is not influenced by the fluid intake system because it is under the control of the velocity of the source water body flow (Vw). Generally, Vw will be constant over short time intervals, but may vary significantly over longer periods of time as a result of a variety of environmental factors (for example, tide, weather, rain, season, etc.).
The probability (p) of an object being entrained/impinged by a filter associated with a fluid intake system is related in a complex manner to the interactions between d, Vi, and Vw. Generally, p is expected to be low if the ratio of Vw/Vi is high. In other words, the likelihood of being entrained/impinged is low if the velocity of the source water body is significantly faster than the velocity of water being drawn into the fluid intake system, because a high ratio of Vw to Vi has the effect of decreasing the value of dmax so there is a smaller distance from the point of intake origin at which Vi can exert an effect that is stronger than Vw. In this situation, entrainment/impingement is likely to occur only if an object happens to pass very close to the opening of the fluid intake system.
There are few examples of anti-entrainment/impingement solutions in the art. U.S. Pat. No. 6,051,131 and U.S. Patent Application 0227962A1 recite the use of wire screens wrapped around an intake source, or slots in an intake pipe, to attempt to filter aquatic life forms from the intake fluid. Disadvantageously, these systems are prone to clogging, and require frequent and costly upkeep to maintain their intake function. U.S. Pat. No. 7,118,307 recites the use of intake pipes covered in wire screens and buried under a natural bed of sand located below the intake fluid source to provide a two pass screening system. Disadvantageously, this system is labor intensive and costly to install, as well as difficult to maintain.
U.S. Pat. No. 5,580,454 (hereafter the “'454 patent”, incorporated by reference herein) discloses a filter cartridge that is backwashable and may provide aspects of a filter element suitable for screening large volume fluid intake systems. However, the filter cartridge of the '454 patent was not heretofore used in such a fluid intake filtering application. On the contrary, the filter cartridge of the '454 patent was designed as an in-line filter for use in high pressure applications; consequently, it has aspects that are not suited for use in screening large volume fluid intake systems. For example, the filter cartridge of the '454 patent was designed for use within a sealed, pressurized vessel (FIG. 1A, 22), and has mounting flanges specific for this type of in-line application (FIG. 1, 24). Additionally, because of the high pressures involved in this filtering application (and correspondingly high values of Vi), the mounting flanges contain narrow diameter fluid connectors for moving the filtrate between the two chambers of the vessel (FIG. 1, 26), and such connectors would not be suitable for an application using lower values of Vi (e.g. cooling water intake for a power plant).